i893. SUCCESSION OF TEETH IN MAMMALS. 363 



The only possible conclusion to be drawn is that in Didelphys 

 (and probably in other Marsupials) the teeth of the permanent 

 dentition, with the exception of the last premolar, represent the 

 deciduous teeth of other forms, while rudiments of a second series 

 are present in the embryo, but only cut the gum in the case of the third 

 premolar ; these rudiments must obviously be the remnants of a more 

 complete replacement. 



If the two series of teeth in the Mammalia are to be regarded as 

 relics of the many series in the Reptilia, the Marsupials, inasmuch 

 as they have almost lost one of these sets, have, in this respect at 

 least, departed further from the common stock of Mammalia than 

 have many of the true placental forms. There is, however, not 

 unfrequently, considerable variation in the extent of replacement, 

 even within the same groups of placental Mammals ; for instance, 

 among the Rodents, the Leporidae have their incisors replaced, while 

 the rat is entirely monophyodont. 



A further point of interest lies in the fact that, whereas in the 

 Marsupials it is the second series of teeth, or that which corresponds 

 with the permanent series of most higher forms, which has become 

 degenerate, in other groups it is the precursory ones that are sup- 

 pressed. We see this suppression of the milk teeth in some of the 

 Carnivora : in the dog, for example, the deciduous teeth are well 

 developed, the bear has much smaller ones, while in the seal they 

 are still more rudimentary, and absorbed before birth. 



E. C. Pollard. 



